Study of the reduction and reoxidation of substoichiometric magnetite

The commercial magnetite powder was characterised as substoichiometric magnetite (Fe2.93O4) with a small hematite fraction (similar to 7 wt%). It consisted of micrometer-sized particles of regular (octahedral) and irregular morphologies. Reference sample was subjected to reduction by hydrogen gas and reoxidation at a temperature of up to 600 degrees C. The oxidation experiments were performed in an oxygen stream. Reference sample oxidised to a maghemite-hematite mixture, whereas the fraction of each phase was very sensitive to the oxidation temperature. XRD and FT-IR spectroscopy indicated the superstructure character of obtained maghemite samples with the ordering of cation vacancies in the maghemite crystal lattice. Mossbauer spectroscopy was very sensitive to magnetite stoichiometry, however, it could not detect less than 10 wt% of hematite in the maghemite-hematite mixture. In reduction experiments, conditions for the reduction of reference sample to stoichiometric magnetite (Fe3.00O4) were found. The reduction was performed by hydrogen gas under static conditions (375 degrees C, 150 min). Under this static condition the relatively high amount of water condensed inside the evacuated quartz tube. The formation of water is due to the chemisorption of hydrogen molecules on the iron oxide, which then dissociate from iron oxide generating an intermediate hydroxyl group according to the general equations: 2O(2-)(s) + H-2(g) -> 2OH(-)(s) + 2e(-) and2OH(-)(s) -> O2-(s) + square(a) + H2O(g), where (s) signifies a solid (hematite or substoichiometric magnetite), (g) a gas phase and square(a) an anionic vacancy formed in hematite or substoichiometric magnetite. Thus, in order to ensure a good reduction condition, water vapour was removed from the quartz tube and the tube was refilled with hydrogen gas every 30 min. (C) 2008 Elsevier B.V. All rights reserved.